The present invention relates to thermoplastic molding compositions that are usefully employed in the preparation of molded objects. More particularly, the present invention relates to a pigmented, opaque, impact modified thermoplastic molding composition having improved colorability.
It is previously well known in the art to prepare impact modified thermoplastic molding compositions by polymerizing one or more copolymerizable monomers in the presence of a rubbery impact modifying polymer. Suitable processes include the well known emulsion polymerization, as well as mass or solution polymerization techniques. In the former, a latex comprising preformed rubber particles is first formed and monomers to be polymerized as the matrix are later added and polymerized to form one or more layers of grafted, hard phase polymer along with a matrix phase of the same monomer composition. One or more than one additional grafted, hard phase polymers may be polymerized.
Alternatively, the rubber modified molding compositions may be prepared by dissolving the rubber polymer in a solution comprising at least some of the monomers to be polymerized and initiating polymerization under conditions so as to result in the formation of the desired rubber morphology. In particular, it is highly desired to cause the polymerizing mixture to phase invert and thereby achieve an occluded and grafted rubber particle morphology.
The impact modified thermoplastic molding compositions prepared according to the foregoing techniques may employ a variety of polymerizable monomers in order to prepare compositions having desired end use properties. Particularly desirable properties of solvent resistance and ease in moldability are obtained where the polymerizable monomers comprise styrene and acrylonitrile. The resulting compositions are referred to as ABS resins where the rubbery polymer employed is a homopolymer or copolymer of butadiene. Such compositions employing an EPDM rubber are referred to as AES resins. Compositions employing an acrylate rubber such as a homopolymer or copolymer of n-butyl acrylate are referred to commonly as ASA resins.
Solvent resistance for a polymer may be determined by the solubility parameter for such resin as is well known in the art and disclosed for example in J. Brandrup, et al. Polymer Handbook, 2nd Ed. Wiley-Intrescience p. IV-337 (1975). Such solubility parameters, .delta., expressed as [cal/cm.sup.3 ]1/2 are calculated from measurements of polymer density, p, in g/cm.sup.3 and polymer weight fraction, .omega., according to the formula: ##EQU1## wherein: .omega..sub.a =weight fraction of acrylonitrile in rigid phase
.omega..sub.m =weight fraction of methylmethacrylate in rigid phase PA0 .rho..sub.ps =density of polystyrene (1.08 g/cm.sup.3) PA0 .rho..sub.pa =density of polyacrylonitrile (1.18 g/cm.sup.3) PA0 .rho..sub.pm =density of polymethylmethacrylate (1.18 g/cm.sup.3) PA0 .delta..sub.ps=solubility parameter of polystyrene PA0 .delta..sub.pm solubility parameter of polymethylemethacrylate PA0 .delta..sub.pa =solubility parameter of polyacrylonitrile
Ideally, the solubility parameter is between the values of 9.90 and 9.70 for styrene and acrylonitrile containing copolymers and most preferable between about 9.85 and 9.75. Within such range the polymers possess exceptional resistance to a wide variety of solvents.
The preceding styrene/acrylonitrile based molding resins have enjoyed enormous commercial success. Such resins are employed in numerous end use applications requiring chemical resistance toughness and ease of moldability. Due to the presence of the particulated rubbery morphology, such resins are inherently opaque or extremely hazy. Such opacity or haze is due to the fact that the differing polymeric phases possess different refractive indices. In order to provide a uniformly colored molded object, it is standard practice to incorporate suitable pigments or colorants into the impact modified thermoplastic molding compositions currently available.
The resulting molded object contains pigment or colorant uniformly and homogeneously dispersed throughout the entire molded object. Because the inherent opacity or haziness of the resin, the large quantities of pigment or colorant located deep in the interior of the molded object do not contribute to the observed color of the final object and in fact represent an unavoidable waste of pigment. For all practical purposes, a pigmented thermoplastic molded object has a color determined solely by the pigment contained in the resin closest to the surface. Thus, it is seen that deep coloration of existing thermoplastic molding compositions requires the use of relatively excessive amounts of pigment since only the pigment contained in a thin layer close to the outer surface effectively contributes to the perceived coloration of the resulting object.
The foregoing may be stated in the alternative as considering polymer pigmentation to involve the additive result of coloration due to the desired pigment combined with the whitening effect due to the rubber particles. Dark color tones require the use of larger amounts of pigment due to the presence of the white rubber particles that act as white pigments. Besides considerations of cost in the use of excessive amounts of pigments it is desirable in order to achieve optimum physical properties in the resulting resin, particularly impact strength and elongation, that reduced amounts of pigment be utilized.
It would be desirable if there were provided a thermoplastic, rubber modified, acrylonitrile containing molding resin having improved colorability. That is, it would be desirable if there were provided a thermoplastic molding resin whereby the same degree of coloration may be obtained by the use of a reduced quantity of pigment compared with a resin wherein the matrix consists of styrene and acrylonitrile only. This ease of colorability of a given resin may be defined by the term "Opacity Factor", which for the present resins is defined as the absolute value of the difference between the refractive index of the matrix and the refractive index of the rubber. Resins having the same opacity factor may be colored to the same degree of coloration (i.e. color matched to the same standard) by use of the same amount of pigment.
An extensive literature exists concerning the preparation of transparent impact resistant thermoplastic molding compositions. Examples are contained in C. B. Bucknall, Toughened Plastics, Applied Science Publishers, Ltd., page 51, and Bower, et al., Resins with Diene Elastomers, Multicomponent Polymer Systems, Advances in Chemistry Series, No. 99, American Chemical Society (1971) at page 247. Such transparent interpolymers typically possess a compositional range of from 11 to 18 percent of a 1,3-polybutadiene rubber, 34 to 39 percent styrene, and 23 to 25 percent each of acrylonitrile and methylmethacrylate.
Obviously, the technique of pigmenting and coloring a resin in order to thereby render such resin opaque is directly contrary to the technique of producing a relatively transparent impact resistance thermoplastic molding composition. Accordingly, for purposes of maintaining opacity to a sufficient degree to prepare solid molded objects which appear visually opaque, it will be readily appreciated that matching of the refractive indices of the matrix and the rubber phase is not suggested by the aforementioned teachings regarding transparent molding compositions.
Additionally, the presence of both methylmethacrylate and acrylonitrile monomers in a copolymer results in instability in the resulting resin. At elevated temperatures such as are encountered in a devolatilizer during recovery of the polymer from a polymerization process, polymer degradation can result thereby producing a color change in the resin. The discoloration or yellowing of the polymer may be quantified by the value, b, according to ASTM E308-85, and for such a yellow shift is always a positive value. The amount of change in the yellowness index (.DELTA.b) is desirably less than 11.0 and preferably less than 9.0 when measured by the above technique, in order to not result in significant yellowness in the final product.